Diabetes is a disorder of metabolism in the way the body uses digested food for growth and energy. From a medical point of view, diabetes is regarded as a disease in which an absolute or relative inadequacy of insulin effect gives rise to a complex disturbance of metabolism that is dominated by an increase of glucose levels and/or intensive lipolysis. Because glucose is not a readily available energy source in the case of insulin deficiency, ketones are produced as an alternative energy source. Ketones, also known in the art as “ketone bodies,” are chemicals that the body makes when there is not enough insulin in the blood. Ketones (acetone, for example) build up in the blood and the body can remove them in the urine. The gas phase acetone in the bloodstream equilibrates with alveolar air (exhaled air) through the alveoli, and the concentration of acetone in breath can reflect metabolic products of diabetes in some way. The body can rid itself of acetone through the lungs, which gives the breath a fruity odor. If a large amount of acetone forms and passes through the body into the blood, urine, and breath, it usually means that the cells either do not have enough insulin, or cannot use the insulin in the proper way. Ketone bodies are produced by the liver and used peripherally as an energy source when glucose is not readily available.
Ketones are always present in the blood and their levels increase with diabetes, which is the most common pathological cause of elevated blood ketones. Ketones include the molecules acetoacetate (AcAc), 3-β-hydroxybutyrate (3HB), and acetone. In diabetic ketoacidosis (DKA), high levels of ketones are produced in response to low insulin levels. As shown in FIG. 1, AcAc accumulates during fatty acid metabolism under low carbohydrate conditions. 3HB is formed from the reduction of AcAc in the mitochondria. These two predominant ketones are energy rich compounds that transfer energy from the liver to other tissues. Acetone is generated by spontaneous decarboxylation of AcAc and is responsible for the sweet odor on the breath of individuals with ketoacidosis.
Diabetes is a large and growing problem throughout the world. The World Health Organization reported that the global diabetic number was 171 million for the year 2000 and assessed the number for the year 2030 would be 336 million. In the United States, 18.2 million people are estimated to have diabetes problems. Persons with diabetes are at increased risk for debilitating complications such as renal failure, blindness, nerve damage, and vascular disease. The number of diabetes cases in the United States is increasing at about 5 percent each year, and ranked as the sixth leading cause of death and disability, at a cost of 132 billion dollars per year. Demand for monitoring and diabetes diagnostic products is projected to advance over 8 percent annually to 4.4 billion dollars in 2008.
Currently available methods for primary screening and diagnostic tests of diabetes are generally inconvenient and unpleasant. Fast Plasma Glucose (FPG), and the Oral Glucose Tolerance Test (OGTT), for example, require venous draws and are fasting-based tests, so they can only be practically administered during morning appointments. For OGTT, the measurement is performed about 2 hours after the patient ingests a 75-gram oral glucose load. Numerous studies have evaluated the performance of these methods in diverse populations. Specificity (correctly identifying absence of disease) of FPG test exceeds 96 percent, but the sensitivity (correctly identifying disease) is only on the order of about 50 percent. Thus, approximately half of those with diabetes may be misclassified by a single FPG test. The sensitivity and specificity for OGTT are 73 percent and 80 percent, respectively, but the test suffers from relatively poor reproducibility (coefficient and variation of about 17 percent). Moreover, glucose self-monitoring is typically done by pricking a finger and extracting a drop of blood with a test strip composed of chemicals sensitive to glucose. In order to strictly control glucose and effectively mitigate the complications, 4-7 tests per day are recommended for diabetes patients. Due to the physical suffering and daily cost, this type of diabetic testing can be carried out usually at a rate of twice a day. Because of all of these restrictions, scientists have been trying to find effective noninvasive ways for diabetes monitoring through measuring blood glucose. For this purpose, a huge sum of funds and manpower have been invested to develop various noninvasive technologies including, for example, Raman spectroscopy, infrared spectroscopy, near infrared spectroscopy, photoacoustic spectroscopy, light scattering, and polarization changes. So far, none of these technologies achieves a satisfactory level for practical applications.
The detection of volatile organic compounds (VOCs) in breath for the purpose of medical diagnosis has a long history. Ancient Greek physicians knew that the aroma of human breath could provide clues to medical diagnosis. The astute physician was alert for the sweet fruity odor of acetone in a patient's breath with uncontrolled diabetes, the musty fishy reek of advanced liver disease, the urine-like smell that accompanies failing kidneys, and the putrid stench of a lung abscess.
Modern breath analysis began in the 1970's when Pauling and others identified more than 200 components in human breath using gas chromatography. Since then, issues concerning the physiological meaning of the breath substances and correlations of breath markers with patients' clinical conditions have become more and more important. Since the 1990's, researchers have been trying to understand the relationship between various breath substances and the body's physical condition. Among the chemicals identified in breath gas, it has been shown that acetone is correlated with blood glucose as an alternative biomarker for diabetes. In healthy individuals, breath acetone is at the level of a few hundred ppbv, whereas diabetics have a broader range of breath acetone concentration of hundreds or even thousands of ppm, depending mainly on physical properties of individuals and the level of blood glucose of patients. The composition of VOCs in breath varies widely from person to person, and includes at least hundreds of compounds. The concentration of acetone in breath can be as low as a few hundred ppbv, making accurate analysis of breath acetone a significant challenge.
A current test for breath acetone is carried out using gas chromatography (GC) coupled with a detection method such as mass spectroscopy (MS) or flame ionization. These methods require bulky equipment and skilled operators, and are time consuming with regard to sample collection, transportation, storage, and separation, and therefore are not suitable for outside laboratory applications such as daily monitoring and prescreening diagnosis of diabetes. In addition, the trace amounts of acetone that are present in the breath may easily be lost during these complex procedures because acetone is a volatile and chemically active material. Some other methods, such as light addressable potentiometric sensors and electronic nose, are less selective for particular components, and therefore, potential interference of the breath matrix gas on acetone detection is unavoidable.
Recently, cavity ringdown spectroscopy was used for detecting acetone at ultraviolet and near-infrared wavelength ranges. Although a detection limit of 1.5 ppmv for acetone vapor was reported, the moisture in breath gas may impact sensitivity because the reflectivity of the cavity mirrors is prone to the humidity in breath, and it is difficult to completely filter out the large amount of water vapor in breath without influencing the trace amount of acetone because acetone is miscible with water in almost any ratio. The added cost of the laser used in this system may be an extra issue for the desirable features in breath analysis.
A reported method for determining acetone in breath is based on the reaction of acetone with alkaline salicylaldehyde to form a colored product that absorbs in the blue region and can be monitored with gallium nitride-based light emitting diodes (LEDs) with an emission centered at 465 nm. The method achieved a detection limit as low as 14 ppbv for acetone. However, memory effects of the reaction system and trace loose acetone are potential problems for the technique in real breath sample analysis.